Prepared for the 51 st Meeting of the USDOE Cross Section Evaluation Working Group at BNL November 5-7, 2001

A Short History of Nuclear Data and Its Evaluation

Last Updated: March 12, 2004

Norman E. Holden*
National Nuclear Data Center
Brookhaven National Laboratory
Upton, New York 11973-5000 USA


This paper reviews both neutron and non-neutron nuclear data over the past century, especially those data of relevance to the USDOE (formerly the USAEC) Cross Section Evaluation Working Group, CSEWG. Among the topics whose history will be examined are neutron cross sections, charged particle cross sections, neutron resonance integrals and neutron fission product yields. Other topics discussed include isotopic composition of the elements, nuclear spins and parities, radioactive half-lives, nuclear magnetic dipole moments and electrical quadrupole moments, alpha particle energies and intensitites, beta-ray energies and intensities and gamma-ray energies and intensities. The status of automation of these parameters into data files will briefly be discussed.


In 1966, the Division of Reactor Development and Technology (DRDT) of the United States Atomic Energy Commission (USAEC) was concerned about the problems involved in the evaluation and processing of nuclear data for reactor calculations. The DRDT's plan for the development of the necessary methods for the processing of the data and for obtaining data for immediate use in reactor calculations involved a long range goal of developing automated methods for processing nuclear data, as well as a short range goal of providing reactor designers with a reference set of data that they could use for their current projects. For the short term goal of obtaining a reference set of nuclear data, the DRDT sponsored the Cross Section Evaluation Working Group (CSEWG), a co-operative evaluation effort aimed at providing reactor designers with a good set of evaluated nuclear data. The proposed long range goal was to be addressed by CSEWG over time. Although the initial effort focused on neutron cross section data primarily, over time, CSEWG added other categories of nuclear data to their automated files of information.

This paper will review the status of the evaluation of nuclear data beginning with the time that radioactivity was first discovered and nuclear data first became available in the late nineteenth century. It will conclude with the time that CSEWG began operation and held its first meeting at the Brookhaven National Laboratory's (BNL's) Cross Section Evaluation Center on June 9th and 10th, 1966. An overal general history of various categories of nuclear data will be followed by a review of the evaluations of specific types of data.


Our history of nuclear data begins at the end of the nineteenth century with the discovery of uranium rays (radioactivity) in February 1896 by Henri Becquerel1. The early part of that century saw the proposal of the atomic theory of matter and the concepts of atomic weights of the elements by John Dalton2. Later in the century, Lothar Meyer and Dmitri Mendeleev studied the physical and chemical properties of the chemical elements, respectively, and by using these properties, they arranged the atomic weights of the then known chemical elements in a periodic fashion, the so-called Periodic Table3.

Toward the end of the nineteenth century, Issac Newton's laws explained the behavior of both objects in motion and of gravity. From the work of James Clerk Maxwell and Heinrich Hertz, it was known that light, electricity and magnetism are interrelated and they were explained by a few simple equations. Everything was thought to be under control. Physicists thought that they had matters in hand. The first American Nobel Prize winner, Albert A. Michelson, in an 1894 speech at the University of Chicago, lamented that "the most important fundamental laws and facts of physical science have all been discovered. These are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote. Our future discoveries must be looked for in the sixth place of decimals."

Within three years of this speech, x-rays were discovered, the electron was discovered and radioactivity was discovered.


Nuclear data began with the discovery of so-called uranic rays by Becquerel and with the work of Marie and Pierre Curie, who tried to determine whether other substances besides uranium also emitted these rays (Marie Curie was the first to refer to this phenomenon as radioactivity4). The understanding of radioactivity was aided by the work of the New Zealand physicist, Ernest Rutherford, and the English chemist, Frederick Soddy on the atomic transformation law, for which Rutherford received the Nobel Prize in Chemistry. This award prompted Rutherford to quip that the quickest transformation he had ever witnessed was his change from physicist to chemists5. The radioactive decay rate was found to be proportional to the number of atoms which were undergoing decay. Mathematically, this led to an exponential form for the decay and to a characteristic decay constant (reciprocal life-time or half-life) associated with each radiation energy.

Although data was to be found in the early publications of the Curies and Rutherford, the first major review of these radioactive data was published in 1931 by the International Standards Radium Commission6 and contained tables of the lifetimes and the radiation energies of the natural radio isotopes. This report was published simultaneously in four other journals around the world; Phys. Zeitschr., J. Am. Chem. Soc., Phil. Mag. and J. Phys. Radium.

In this age of radioactivity at the beginning of the century, many radioactive substances were being found with various atomic weight values. In 1911, Frederick Soddy used his displacement law for a-particle decay (reduce the atomic mass number by four and the atomic charge by two) and (3 transitions (no mass change and increase the charge by one) to show the chemical identity of meso-thorium (228Ra) and radium (226Ra)7. In 1913, he concluded that there were chemical elements with different radioactive properties and different atomic weights but with the same chemical properties, so they occupied the same position in the Periodic Table. He coined the word "isotope" (Greek for iri the same place) to account for these radioactive species8.

In 1897, the English physicist, J. J. Thomson, discovered the electron9. In 1912, he studied the rare gas neon by sending electrons through neon gas, creating neon ions, which he accelerated toward his detector (a photographic plate10). Using electric and magnetic fields operating at right angles to each other to deflect these ions, Thomson found darkening at two separate locations on the photographic plate, corresponding to positions for the 20Ne ion and for the 22Ne ion. Relative intensity of darkening on the photographic plate was 90% and 10%, respectively, for the two ion beams. Given that the atomic weight is equal to the average mass of the element, this could now account for neon's non-integral atomic weight value of 20.2. This was also the first time that isotopes of a stable chemical element had been found, in contrast to all previous isotopes for the radioactive elements. One of Thomson's students, Francis Aston, using a mass spectrograph (a variation of Thomson's instrument), began measuring the percentage of each element's isotopes, or the chemical element's isotopic composition. These percentages of an isotope in a chemical element are now referred to as isotopic abundance values. Aston's first compilation of isotopic abundance values was published in his book on "Isotopes" in 192211.

Using his mass spectrograph, Aston12 observed small divergences of atomic masses from integral values. This led to the use of that instrument for the measurement of atomic masses. A difference between the isotopic mass and the mass number was called the packing fraction. It is related to the binding energy per nucleon in the nucleus. The binding energy of a nucleus is the difference between the combined masses of the nucleons in the nucleus and the mass of the nucleus itself. Measurement of various atomic masses indicated that the binding energy per nucleon varied as a mass number increased. A table of packing fractions appeared in Aston's book on "Mass Spectra and Isotopes" in 193313.

In 1924, Wolfgang Pauli14 postulated the existence of a quantum number, I, of a nucleus (referring to the total nuclear angular momentum or nuclear spin) and the associated magnetic dipole moment, μ. Samuel Goudsmit and Ernst Back15 experimentally verified Pauli's concept of nuclear spin and magnetic moment to explain observed hyperfine structure in spectral lines. In 1930, Linus Pauling and Goudsmit16 produced the first table of nuclear moments in their book on line spectra.


As the 1930s began, the nucleus was thought to contain both protons and electrons to explain the beta decay of natural radio isotopes. In a 1921 public lecture, Ernest Rutherford17 said that there must exist in nature a particle with the same mass as a hydrogen atom but with a zero electric charge, to explain the phenomenon of radioactivity. Frederick Joliot never bothered to read Rutherford's remarks because Joliot wrongly assumed that a public lecture would contain a display of oratory but no new ideas18. Theoretical work was never highly regarded in Marie Curie's laboratory, where Joliot worked. Curie once responded to a theoretical physicist's recommendation that a particular experiment be performed with a comment that they might even perform the experiment in spite of that suggestion19. Rutherford and his assistant, James Chadwick, spent ten years trying to find the neutron without success.

In 1930, Walther Bothe and H. Becker20 observed the emission of a penetrating secondary radiation by various light elements bombarded with polonium α-particles and interpreted it in terms of high energy γ-rays. It was later proved by the same authors that this secondary radiation included a high energy γ-ray component and was due to residual nuclei being left in an excited state. In any case, their geiger point counter was not capable of detecting neutral particles.

Following up on Bothe and Becker's investigation in 1932, Frederick Joliot and his wife, Irene Joliot-Curie, reported that a particles from polonium bombarded beryllium and they produced radiation that knocked protons out of hydrogen atoms21. They thought that they had observed a g­ray Compton effect but Chadwick determined that these γ-rays would require an energy in the range of 55 to 90 million-electron-volts (MeV). He proposed the elusive neutron as the solution and experimentally verified this assumption. Chadwick is now credited with the discovery of the neutron22.

In 1934, Joliot and Joliot-Curie23 determined that the product atom of an artificial disintegration need not always correspond to a stable isotope but could disintegrate with the emission of light particles with a definitive half-life. They had discovered artificial radioactivity. Ernest Lawrence observed the same phenomenon using his cyclotron. He found that his counters "mis-behaved" after the cyclotron was shut off. He "solved" this mal-function by automatically shutting off his counters, once the cyclotron was shut off and never realized that he had missed a fundamental discovery24.

Joliot's production of artificial radioactivity using α particle bombardment only worked with the light chemical elements. The coulomb barrier prevented a nuclear reaction from occurring in a heavy nucleus. Enrico Fermi put together the ideas of the neutron and artificial radioactivity and produced neutron fission.

Most scientists knew that neutron sources were very much weaker than α particle sources. However, Fermi realized that neutrally charged neutrons would be much more effective than the α particles for the study of artificial radioactivity. He proposed irradiating all elements, even heavy ones with neutrons to study artificial radioactivity.

In a series of 1934 papers25, Fermi's group reported irradiating all available elements up to uranium with neutrons and the resulting production of radiations with characteristic half-lives. He thought that they had created26 element 93 and possibly 94, which they would call ausenium and hesperium27. Fermi sent preprints to 40 prominent nuclear physicists. At the time, Fermi was best known for his theory explaining (3 decay. Rutherford thanked Fenni for the preprint and he congratulated Fermi on his "escape" from theoretical physics28.

Reports29 began to appear in the literature claiming that Fermi's radiation might be protactinium (element 91) and not element 93. Lise Meitner, who had discovered protactinium, convinced her co-discoverer, Otto Hahn to investigate this problem30. In 1934, Ida Tacke Noddack, the discoverer of the element rhenium, published an article31 stating that bombardment of heavy elements with neutrons might cause the nuclei to break into larger pieces, which are isotopes of known chemical elements but not neighbors of those irradiated. No one took this concept of nuclear fission seriously, since it was incompatible with the known laws of physics at the time. It was considered to be pure speculation. Finally in January 1939, the joint efforts of Meitner, Hahn and Fritz Strassman resulted in the publication of the discovery of neutron fission32.

The reasons why the discovery of fission was delayed for five years is also an interesting story33. However, contrast the period from 1934 to 1939 from the time of the first detection of neutron fission until there was an understanding of what had physically taken place, to the much shorter time period from 1939 to 1942 between the actual discovery of neutron fission until the first application of fission in the operation of a sustained man made nuclear chain reaction, in the Chicago Pile (CP-1).

Many groups around the world began scientific work on neutron fission. In the year between the 1939 discovery of fission and a 1940 review article34 by Louis Turner on Nuclear Fission, more than one hundred papers were published and about fifty radioactive fission products had been discovered and partially identified. By September 1939, the Second World War had begun and by 1941, many scientists in the USA, Canada and England imposed a voluntary censorship on their own publications of work in the field of neutron fission. The whole subject became "classified" and virtually disappeared from the literature35. This attempt not to alert the wartime opponents of the interest in nuclear fission was noted by the Japanese, when the hotest scientific subject of the day completely disappeared from the literature36 . Similarly Georgi Flerov, who was on leave from the War Front observed the sudden absence of papers on fission and immediately wrote a letter to Comrade Stalin urging him to set up a uranium committee37.

At the conclusion of the War, many of the previously classified papers were finally published in the open literature including a report38 on the measured data on fission product yields for the neutron fission in 235U. This report was the Plutonium Project Record of the Manhattan Project Technical Series. Studies had now been made directly or indirectly on nearly all of the 160 radioactive fission products then known. The results of the Plutonium Project were presented in tabular form. Nuclear fission product mass chains were divided into a light group (equal to or less than mass 117) and a heavy group (greater than mass 117). Nuclear fission yields were listed for the various mass numbers.


There have been a number of tables of decay data presented in various formats over the years. In 1936 and 1937, Hans Bethe wrote a series of three articles on Nuclear Physics dealing with stationary states of nuclei, nuclear dynamics - theoretical and nuclear dynamics - experimental. In the later article39, Bethe and Stanley Livingston presented tables of reactions and a table of induced radioactivities. In the radioactivity table, they listed nuclide, half-life, radiation and energy, method of production and the references. In 1940, J. J. Livingood and Glenn Seaborg40 collected information on all nuclear species which were produced artificially, following the Livingston and Bethe's model. There was a separate table of stable nuclei with their isotopic abundances and a table of induced radioactivities with half-life, radiation, production methods and references. Later editions of this table included information on all nuclei in one table and was known as the "Table of Isotopes". The sixth edition of this Table41 appeared in the year following the creation of CSEWG.

A member of Fermi's group, Emilio Segre, introduced a scheme for presenting data on all known nuclides in a chart form, where he represented the neutron number, N, in horizontal rows along the left side of the chart and the atomic number, Z, as vertical columns along the bottom of the chart. A revised edition of Segre's Chart from May 15, 1945 was published with classified data omitted42. In 1948, Gerhart Friedlander and Morris Perlman43, at the General Electric Research Laboratory in Schenectady, New York, inverted Segre's chart and plotted the atomic number, Z, against the neutron number, N. This GE Chart of the Nuclides as it would be later called included information on isotopic abundances, radioactive half-lives, radiation type, energies, atomic masses and thermal neutron cross section values. The generation of data for this wall chart was moved to the General Electric's Knolls Atomic Power Laboratory, also in Schenectady and a ninth edition44 of this GE Chart was published at the time of CSEWG's start.

Katharine (Kay) Way had been a member of the Manhattan Project effort working at the Clinton Lab (later renamed Oak Ridge National Lab). While at Oak Ridge after World War II, Kay Way began collecting information on nuclear data. In 1948, Way headed the Nuclear Data Project which was established at the US National Bureau of Standards (later renamed the US National Institute of Standards and Technology). A report45 was published in 1950. The data included measured values with references of isotopic abundances, neutron cross sections, decay modes, conversion coefficients, energies of radiations, genetic relationships, radioactive half-lives, intensities of radiation and methods of production with some reaction energies. There were some decay schemes drawn but there were no recommended values and no errors presented.

In 1953, the Nuclear Data Project was moved to the National Academy of Sciences - National Research Council in Washington, DC. The published data46 now also included coincidence measurements, mass assignments, neutron and proton separation energies, total disintegration energies and spins, magnetic and electric moments. Errors were now listed along with a single decay scheme for all isobars of a given mass number, A. These data were in the form of loose leaf pages called "Nuclear Data Sheets", one for each mass number.

In January 1964, the Nuclear Data Project moved again back to the Oak Ridge National Laboratory, where Kay Way had originally begun her efforts. The Nuclear Data Sheets were once again to be published in a book form by Academic Press, rather than the single sheets of data.

There were additional efforts, most of which were not as comprehensive as the Table of Isotopes, the Chart of the Nuclides or the Nuclear Data Project. These included work in the USSR by Dzelepov beginning in 1950 and later revisions47, reviews of light nuclei energy levels by Tom Lauritsen48 and various co-wokers, including Fay Ajzenberg, who continued this work after Lauritsen's death, and by Pieter Endt49 and his co-workers at the State University of Utrecht, in the Netherlands.


In Aston's 1933 book13, the mass and abundance tables were made up of mass spectrographic measurements (utilizing a photographic plate for ion detection) of packing fractions and isotopic abundances. The atomic masses were determined by the "doublet method"; the mass difference between two atom or molecular ion fragments having the same mass number is determined, where the difference between the mass number and the atomic mass is related to the packing fraction.

At the beginning of the nineteenth century, the first standard for atomic weights50 was hydrogen with a unit value. However, this standard led to the situation where the heavy elements, thorium had a mass of 230 and not 232 and uranium had a mass of 236, not 238. At the beginning of the twentieth century, the use of a standard of oxygen having a value of 16 corrected this problem. In 1929, the isotopes of oxygen were discovered51,52. Chemists continued to use the standard of atomic oxygen = 16, while physicists used a standard of 16O = 16. In 1935, Malcolm Dole53 discovered the variation of the oxygen isotopes in air and in water. This led to a small variable mass difference between the chemist's mass stanard and the physicist's mass standard.

Chemists refused to accept the mass spectrometrists 160 = 16 standard because there were literally tens of thousands of chemical measurements in the literature with uncertainties quoted to better than two tenths of one percent (0.2%), which would be affected. Finally after two decades, Alfred Nier50 provided an acceptable compromise for the standard of mass, that of 12C = 12. With all of the various hydro-carbon compounds available, 12C had been a mass spectrometry secondary standard. This change in the standard to 12C = 12 involved a difference of only 0.004% and was acceptable to the chemists, since it hardly impacted any data in the chemical literature at the time. The International Union of Pure and Applied Physics (IUPAP) approved the mass change at their 1960 General Assembly meeting in Ottawa, Canada and the International Union of Pure and Applied Chemistry (IUPAC) also approved it at their 1961 General Assembly meeting in Montreal, Canada54.

The new 1960 relative nuclidic mass table based on the 12C = 12 scale was published in 1960 by Josef Mattauch and Aaldert Wapstra55. The consistent set of nuclidic masses were computed with least squares methods from all significant experimental data for the mass numbers less than 200.

There were not enough experimental data to perfonn a least squares fit for the data above A > 200. In addition, Al Nier56 had just published his results for atomic masses in the heavy mass region at the same time and these data were available too late in the process to be incorporated into the 1960 mass table.

As a result in 1965, Mattauch and Wapstra57 published the 1964 Atomic Mass Table using a new computer progress on the IBM-7090 calculating best values from a least sqaures fit of data and a χ2 fit of adjusted values. This constituted the latest mass data at the time of the CSEWG meeting.

The original work on isotopic abundances of the elements was perfoiined by Aston using a mass spectrograph and photographic plate detector. By the 1930's, use of the mass spectrometer with electronic detection of the ion beams, especially with those built by Al Nier, allowed Nier to detect minor isotopes for the first time, such as 40K, 36S, 46Ca, 48Ca and 1840s. After the discovery of neutron fission, at Fermi's request, Nier separated 235U from natural uranium, which allowed experimenters to detemnine that it was the 235U isotope and not the more abundant 238U isotope that was causing the fission of thermal neutrons.

In 1950, a report by Kenneth Bainbridge and Al Nier58 provided a complete compilation of all isotopic abundance measurements with comments. Nier would later update tables of isotopic abundances.


It was mentioned above that Pauli introduced the concept of nuclear spin to explain the hyperfine splitting of spectral lines in the time frame before the discovery of the neutron. The angular momentum is always conserved in nuclear transitions, so the vector difference between the initial value of spin and the final value must be possessed by a particle absorbed or emitted in the transition.

Parity of a system of particles has no simple analogue in classical mechanics but is a fundamental property of the motion according to quantum mechanics. In quantun mechanics, the absolute value of the wave function, ψψ*, must be the same at the co-ordinate point (x,y,z) as at (-x,-y,-z). If the reflection of the particle through the origin does not change the sign of ψ, the motion of the particle is said to have even parity. If the reflection changes the sign of ψ, the motion of the particle is said to have odd parity. If the orbital angular momentum is even, the reflection does not change and the parity is even, while if the orbital angular momentum is odd, the parity is odd.

Data on the spin and parity of various nuclear ground states and excited energy states have been collected in the various Nuclear Tables, Nuclear Charts and Nuclear Decay Schemes that have been mentioned above.

Similarly, values of the magnetic dipole moment and electric quadrupole moment of nuclei have been collected in the various Nuclear Tables, Nuclear Charts and Nuclear Decay Schemes that have been mentioned above.


Following the discovery of the neutron by Chadwick and the use of the neutron for causing nuclear reactions by Fermi, there were a series of measurements performed of the probability of a neutron to cause a particular reaction. This probability was called a cross section and the general size of the unit of cross section was 10-24 to 10-30 cm2. Somewhere in the time frame of 1941 to 1942, physicists from Purdue University are credited with introducing the term "barn" (with the symbol b) for 10-24 cm2, to describe cross sections that were relatively easy to measure ("as big as a barn")59. The term came into general use in the open literature around 1947. The subunits of milli-barns (mb) for a cross section of 10-27 cm2 and micro-barns (μb) for a cross section of 10-30 cm2 would also eventually appear in the literature.

The Manhattan District Project was the code name used during World War II to refer to all of the wartime work on the American attempt to produce an atomic bomb. A collection of neutron cross sections of the elements based on prewar and wartime work during the Manhattan Project was made by Hyman Goldsmith (BNL) and Herb Ibser (Wisconsin) and it was revised by Bernard Feld (MIT) and Goldsmith and published60 in 1947. Katherine (Kay) Way and G. Haines published61 a series of thermal neutron cross section review tables in the 1947 and 1948 timeframe. By the early 1950s, the first of a series of "barn books" of neutron cross sections were published by the USAEC Neutron Cross Section Advisory Group62 with the designation AECU­2040 in 1952 to 1954. Brookhaven Neutron Cross Section Compilation Group published a series of neutron cross sections reviews, BNL-170, BNL-250 and finally BNL-325, from 1952 to 1955. Later editions of this barn book kept the designation BNL-325 in the 1957, 1958, 1960, 1964, 1965 and 1966 editions and supplements. In addition to compiling the experimental data points, there were hand-drawn best fit curves through the data points, as eye guides. The 1956 publication63 of angular distributions of the cross sections were designated as BNL-400, which designation was also kept in later editions. CENTRAL INTELLIGENCE, NUCLEAR DATA CORPORATION OF AMERICA

As mentioned above, during the World War II, the Manhattan District Project was formed to provide support for the building of the first atomic bomb. Hyman H. Goldsmith was the neutron cross section information coordinator for this Project. Goldsmith is reported to have continually traveled around the country visiting one laboratory of the Project after another and he provided inter-laboratory information exchange by carrying the latest neutron cross section measurement results on various pieces of paper in his pockets.

In 1956, Herbert Goldstein was working at the Nuclear Development Corporation of America (NDA) and he developed a scheme for keeping track of neutron cross section measurements in the bibliographic sense. He called64 his IBM punched card index, "Central Intelligence - NDA", or CINDA. These cards indexed both the published and the unpublished literature on microscopic neutron cross section measurements in a form so that searching for information, keeping the index up to date and providing periodic cumulations could be done quickly and mechanically.

By 1963, the lack of external financial support caused the index to become out of date. Goldstein65 renamed his bibliographic effort "Card Index of Neutron Data" with the same acronym and he solicted external readers to cover the major journal publications and bring CINDA back up to date. With the eventual demise of the punched IBM cards for computer input, the same acronym has evolved to "Computer Index for Neutron Data" and is now performed via a four nuclear data center agreement.


In 1951, the compiling of neutron data started at Brookhaven as a supplemental activity to the neutron measurment program in the Physics Department by Don Hughes, who had come from the Metallurgical Laboratory or Met Lab (later renamed Argonne National Laboratory) in Chicago. When Hughes died in 1960, the Neutron Cross Section Compilation Group (Sigma Center) was moved to the Nuclear Engineering Department. The Cross Section Evaluation Center, CSEC, was organized at this time and in 1967, the Sigma Center and CSEC were merged into the National Neutron Cross Section Center at BNL. In the early 1960s, a major effort was undertaken to place all of the data on magnetic tape in the Sigma Center Information Storage and Retrieval System, SCISRS. One major disadvantage of SCISRS was that it was written in machine language for use at BNL. This was not as useful to other labs who were interested in receiving the data but who did not use the same machine language.

In the mid 1960s, the situation of nuclear data compilation and evaluation was as follows. The next generation of main frame computers (faster calculational speed and larger memory capacity) were beginning to become available at the various reactor design laboratories around the country. The IBM 704, 7090, 7044 and 7094 computers were now being replaced by the Control Data Corporation's (CDC) 6600 computer. This would be exploited in the near future.

At the 1961 Vienna Conference on the Physics of Fast and Intermediate Reactors, Ken Parker66 (Atomic Energy Research Establishment, Aldermaston, UK) indicated some of the requirements for the neutron cross section libraries. These libraries had to specify all of the reaction processes available or else a zero value cross section would automatically be assumed by the computer program. There had to be a simple presentation of the data on punched cards, which would be easy to revise. However, the data could not be revised frequently because in that case the reactor designers would be unable to perform comparative calculation as they made their design revisions. There was a need to cross check the data for errors and the best data should provide reasonable answers for simple systems, such as bare reactor cores. Parker would make a distinction between the compilation of neutron cross section data and the evaluation of neutron cross section data.

At the 1964 Geneva Conference on the Peaceful Uses of Atomic Energy, John Story67 (Atomic Energy Establishment, Winfrith, UK) stated that the uncertainties in neutron cross section data were larger than were being estimated at that time. He defined a cross section data file as a complete set of evaluated cross section data for a single material and a cross section data library as data files for a number of materials. He agreed with Parker that all cross sections must be represented over the full energy range but Story stated that the accuracy need not be the same for 1) all materials in the library, 2) for different cross section reaction types in the library, or 3) for different regions of the energy range in the library.

Story listed the procedure for a data evaluator to follow: 1) search the scientific literature for the cross section data; 2) study the references found in this search and put the cross section data into tables or on to punched cards and compare the resulting data with theory; 3) prepare a set of recommended cross section values on punched cards and check recommended data on the punched cards; and 4) document the details and justify the recommended data.

At the AEC-ENEA Seminar on Cross Section Evaluation at BNL (May 1965), Bob Howerton68 (Lawrence Radiation Laboratory-LRL-Livermore) indicated that early (1957) neutron cross section data provided no associated experimental errors. For many elements and isotopes, there were no evaluated neutron cross sections over a defined range. By 1960, LRL would provide such evaluated data on magnetic tape.

Ken Parker69 at the 1966 Washington Conference also commented on the very large amount of neutron cross section data that was then becoming available due to better machines to generate the data, more experimenters to perform the measurements and new techniques for automatic computer handling of numerical data. The data evaluators were being overwhelmed. The only solution was to increase the evaluators output by introducing computer mechanized evaluation of the cross sections. The majority of the effort of cross section data evaluation was devoted to the collection, the plotting and the tabulating of the experimental and theoretical data with a minority of the time on the actual evaluation of the data. The collection of all relevant numerical data on SCISRS tape was a start in the right direction but there was still the problem of the data being represented in machine language.

In 1964, Henry Honeck at Brookhaven began work on the Evaluated Neutron Data File (ENDF) as a vehicle to simplify the exchange of evaluated data. It would serve as a link between a data library and the processing codes. ENDF would allow data sets from different sources to be placed in a common format for use in neutronic calculations. Once it is created, CSEWG will generate and test new and revised data for the ENDF/B library.

In the early CSEWG days (1968), Herb Goldstein70 would comment about the use of neutron transport programs that had built-in neutron cross section libraries that could not be modified. As a result, many neutron cross section measurers might be shocked to see the recent data that they had measured and which had been available for over a half decade could not be made use of by the reactor designers.

In 1966, Ken Parker69 had commented that the rules for selection of data are either logical, in which case they could in principle be used by a computer, or else they are illogical, in which case they should not be used at all. However, by 1968, Herb Kouts71 would comment that attempted machine made evaluation programs such as SCORE (from Atomics International) could not replace an experienced neutron cross section evaluator such as Joe Schmidt (Karlsruhe) in Kouts' estimation.


From the above review of nuclear data evaluation over the past century compared to the situation in the present day, the vast amount of change that has been wrought by the work of the cross section evaluation working group can be seen in both the areas of nuclear data evaluation as well as the automation of the data files.


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*This research was carried out under the auspices of the US Department of Energy, Contract Number DE-AC02-98CH10886

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